Enhancing Corrosion Resistance: The Synergy of Coatings and Cathodic Protection

Enhancing Corrosion Resistance: The Synergy of Coatings and Cathodic Protection

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The Benefits of Combining Coatings with Cathodic Protection

Coatings and cathodic protection are often used in conjunction with each other when designing and implementing corrosion protection systems. But why?

Corrosion is one of the most significant challenges in maintaining the integrity of structures, installations, components, and mechanical systems. It involves the deterioration of materials (primarily metals) through chemical reactions with their environment, leading to the formation of oxides, hydroxides, or sulphides. The result is compromised structural integrity and functionality of critical systems.

The factors that influence corrosion include environmental conditions, material properties, and electrochemical activity, with the presence of an active metal surface being a crucial element in the corrosion process.

Mechanisms of Corrosion Protection

To combat corrosion, it is essential to transform active metal surfaces into passive ones. This process can be achieved by reducing the potential of the metal surface through the application of a protective current, a technique known as cathodic protection.

Cathodic protection effectively halts or significantly slows down electrochemical changes, preventing corrosive attacks. How does cathodic protection work? There are two primary methods of implementing cathodic protection:

1.    Sacrificial Anode Cathodic Protection (SACP)

This method involves attaching a more easily corroded ‘sacrificial’ metal to the protected metal. The sacrificial anode corrodes in place of the protected metal.

2.    Impressed Current Cathodic Protection (ICCP)

In this method, an external power source supplies a current to the protected metal, countering the corrosive current and thus preventing corrosion.

The Role of Coatings in Corrosion Prevention

Coatings serve as another effective strategy for mitigating corrosion. They function as a protective layer, barrier, or sacrificial layer over the metal surface. These coatings enhance the material’s surface characteristics, protect against corrosive environments, and reduce the risk of failure.

Coatings are particularly beneficial in environments where structures are exposed to dynamic loads, such as marine structures, where they help prevent corrosion fatigue and stress corrosion cracking. However, in extremely aggressive environments, the use of coatings alone might not be sufficient.

The Synergy of Coatings and Cathodic Protection

Combining coatings with cathodic protection offers a robust solution for enhancing metal resistance against corrosion. This dual approach provides several benefits, including:

·       Increased Durability

The combination extends the operational life of structures and components by providing multiple layers of defence against corrosion.

·       Cost-Efficiency

It reduces the need for frequent maintenance and repairs, thus lowering overall lifecycle costs.

·       Design Optimisation

By mitigating corrosion risks, it allows for the use of lighter materials, reducing fabrication and transport costs, and contributing to sustainability goals.

Advanced Coating Technologies

Recent advances in coating technologies have led to the development of nanocoatings and nanocomposite coatings, which offer superior protection in harsh and complex environments. Examples of these advanced coatings include Nickel-Alumina (Ni/Al2O3), Nickel-Silicon Carbide (Ni/SiC), Nickel-Zirconia (Ni/ZrO2), and Ni-Graphene (GPL). These nanocomposite coatings are designed to withstand both mechanical wear and chemical corrosion, providing enhanced protection for components in demanding applications.

These advanced coatings have been subjected to rigorous testing, including ASTM B117 salt spray testing, to validate their performance under corrosive conditions.

The Bottom Line

In summary, the primary objective of industrial coatings is to prevent corrosion and withstand hazardous chemicals. Selecting the right coating and supplementing it with cathodic protection can significantly enhance the durability and reliability of structures.

Advanced coating technologies and remote monitoring systems further improve corrosion resistance, offering bespoke solutions for specific applications.

By integrating these strategies, industries can achieve cost-effective, efficient, and reliable protection against corrosion, ensuring the longevity and safety of critical infrastructure, and achieving significant cost savings and environmental benefits.

What Do You Want to Ask an Expert?

This article has been adapted from a question that was sent to the Institute of Corrosion and answered by Professor Zulfiqar Khan. The question was, “Why are coatings often supplemented with cathodic protection to protect against corrosion?”, and we first published Professor Khan’s answer in our exclusive Corrosion Management magazine.

If you have a question to pose to our experts, please feel free to send it to us by email, and we’ll do our best to have the answer published for you.

Effective Strategies for Preventing Corrosion of Reinforcing Bars in Concrete

Effective Strategies for Preventing Corrosion of Reinforcing Bars in Concrete

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Corrosion of reinforcing bars in concrete can significantly compromise the structural integrity and longevity of concrete structures. Inadequate attention to this phenomenon can have many consequences. Not only can it lead to extraordinary costs to repair and maintain concrete infrastructure, but, in the worst-case scenario, collapse of buildings and bridges can cause environmental issues and lead to loss of life.

Addressing this issue requires a combination of proper design, construction practices, and maintenance strategies.

Ensuring Proper Design and Construction

The most effective way to prevent corrosion of reinforcing bars is to ensure that the design and construction phases are executed correctly. This involves achieving the required depth and quality of cover for the reinforcement.

Common issues leading to corrosion include:

  • Poor design
  • Inadequate detailing
  • Lack of control over the concrete mix
  • Improper placement and curing
  • Mis-location of reinforcement resulting in insufficient cover

Portland cement-based concretes inherently protect reinforcement by creating a low permeability and highly alkaline environment. This environment allows the steel to form a stable passive oxide film, which acts as a protective layer.

Maintaining suitable conditions is crucial for the steel’s protection. The concrete cover serves as a thick barrier that not only physically protects the steel but also contains chemical species that contribute to this protection. During the initial stages, the concrete cover can even self-heal minor cracks caused by shrinkage as it cures and reaches full strength.

Addressing Chloride Ions and Carbonation

Two primary initiators of reinforcement corrosion are chloride ions and carbonation.

Chloride ions can cause severe pitting corrosion even in highly alkaline environments. They can enter the concrete mix through accidental contamination, purposeful addition (historically, calcium chloride was used as a set accelerator), or from external sources such as marine environments and de-icing salts.

Carbonation occurs when carbon dioxide from the atmosphere enters the concrete cover and dissolves in the moisture present, forming carbonic acid. This acid neutralizes the alkalinity of the cement, reducing the protective environment for the steel. In high-quality concrete with adequate cover depth, carbonation progresses slowly, taking decades or even centuries to reach the steel. However, poor-quality concrete or insufficient cover depth can lead to the steel losing its protection within a few years, resulting in corrosion in the presence of moisture.

Current Standards and Practices

Modern standards and codes of practice provide comprehensive guidelines to mitigate corrosion risks from chlorides and carbonation under various exposure conditions. For particularly aggressive environments, such as offshore applications, additional protective measures are necessary. These measures also apply to existing structures where durability has been compromised due to historical design or construction shortcomings.

Protective Coatings and Repair Techniques

Applying coatings to the concrete surface is a widely adopted method to resist chloride ingress and carbonation, extending the service life of existing structures. Periodic recoating can further extend the protection duration. In cases where reinforcement corrosion has already occurred, removing loose concrete, cleaning the steel, and reinstating with fresh concrete (often modified repair mortar) can be effective, particularly for carbonated or mechanically damaged concrete with low chloride levels.

For chloride-contaminated concrete, complete removal of residual chloride is impractical. Instead, corrosion inhibitors or cathodic protection are employed. Corrosion inhibitors can be applied to the concrete surface or incorporated into the repair mortar to enhance its resistance to chlorides. When chloride levels are too high for inhibitors to be effective, or when a long service life extension is needed, cathodic protection becomes the viable alternative.

Cathodic Protection

Cathodic protection (CP), a technique dating back to 1824, is extensively used to protect buried or submerged structures such as pipelines and offshore facilities. In recent decades, CP has also been employed for reinforcing steel in concrete. This method involves creating an electrochemical cell that prevents the steel from corroding. CP systems require specialized design and should be installed by personnel certified to ISO 15257: 2017 standards.

The Bottom Line

Preventing corrosion of reinforcing bars in concrete primarily involves ensuring adequate design and construction practices to maintain a high-quality, alkaline, chloride-free environment. When this is not feasible, additional measures such as surface coatings, corrosion inhibitors, and cathodic protection are essential to control corrosion and extend the lifespan of concrete structures. Through diligent application of these strategies, the durability and integrity of reinforced concrete structures can be significantly enhanced.

What do You Want to Ask an Expert?

This article has been adapted from a question that was sent to the Institute of Corrosion and answered by Paul Lambert. The question was “What is the best way to prevent corrosion of reinforcing bars in concrete?”, and we first published Paul’s answer in our exclusive Corrosion Management magazine.

If you have a question to pose to our experts, please feel free to send it to us by email, and we’ll do our best to have the answer published for you.